This invention relates generally to laser discharge tube assemblies and more particularly to tube assemblies of such type which are adapted for use in ring laser gyroscopes.
As is known in the art, a laser discharge tube assembly may include an elongated capillary bounded on each end by a laser window or reflector. An active medium such as a helium -- neon gas mixture is contained within the capillary of such assembly. An anode electrode and a cathode electrode are provided to cause an electrical discharge through the gas mixture in response to a suitable voltage. By including such assembly and a polarization dispersing structure in a "ring" shaped or closed resonator, a ring laser gyroscope, as described in my U.S. Pat. No. 3,741,657, issued June 26, 1973 and assigned to the same assignee as my present invention, may be constructed.
As is further known in the art, the gain provided by such laser discharge tube assembly is approximately proportional to the ratio of the length of the capillary to the diameter of such capillary. Hence, in many applications requiring relatively large gain in a relatively small space, such as a practical ring laser gyroscope application, it is generally desirable that the just mentioned ratio be maximized.
Laser discharge tube assemblies of the type described above are generally fabricated by using either glass blowing or by drilling a hole in a solid block of a suitable vitreous material to form the desired capillary. While such fabrication techniques have been found satisfactory in some applications, such techniques are generally inadequate when used in laser gyroscopes where the capillary length in a given laser discharge tube is increased by using a folded, rather than a straight, capillary. Such folded capillary, for example, may take the form of a V-shape, or a W-shape, or any other discontinuous shape comprised of sections of straight capillary segments with a reflective means, such as a mirror, disposed at each point of inflection formed by pairs of adjacent capillary segments. With such folded capillary it is generally desirable that each capillary segment have a length to diameter ratio in the order of 50:1 when the diameter is in the order of 1 millimeter. Further, in order to minimize so-called "aperture effects" or interference from the walls of each capillary segment, and hence maximize the effectiveness of the gyroscope, it is generally desirable that such capillary segment be maintained straight to a maximum deviation in the order of .+-. 0.05 millimeter over a 50 millimeter length and also that the angles of incidence and reflection at each mirror be equal to within the order of two minutes of arc. Still further, it is generally desirable that the point of intersection of any two adjacent capillary segments be in the plane of a corresponding one of the mirrors within a tolerance in the order of .+-. 0.05 millimeter and also that each capillary segment be disposed in a common plane. The aforementioned tolerances are not readily attainable using conventional glass blowing or hole drilling technology.
Another difficulty which may be encountered in forming any capillary using the above-mentioned drilling fabrication technique is that rough surfaces of the capillary wall may be formed. If a quartz block were used, for example, the discharge through the capillary would then tend to cause erosion of "high spots" or peaks making up the rough wall surfaces, thereby resulting in contamination of the helium -- neon gas mixture with such impurities as silicon dioxide or oxygen. The final effect of such contamination is to reduce the operating life of the gyroscope. It also follows that such a fabrication technique does not generally readily enable processing, as polishing, of the capillary walls to thereby smooth the capillary wall surfaces and hence reduce the amount of expected contamination and thereby extend the operating life of the gyroscope.